The cell
survival curve is the plot of log surviving fraction
versus dose...

How is
a cell survival curve established?

Visualize a set of Petri plates each
having a known number of clonogenic cells in an environment that will
support cell proliferation. A control dish receives no radiation.
Successive plates receive increasing doses of X-rays. The plates are
incubated until each surviving cell (clonagen) has
produced a macroscopic colony. The colonies on each plate are counted.

For each dose, a surviving fraction, S, may be calculated
as: the number of
colonies receiving the dose divided by the number of colonies on the
control
plate.

Cell
survival curves for neutrons and x-rays. In the above example, the
biologic effect is surviving fraction of cell colonies. X-rays of 250
KeV are the usual standard for comparison – so, RBE may be
read as “the dose multiplier for 250 KeV x-rays to give
identical biologic effect.” In most situations, the
cell survival curve can be modeled as a linear-quadratic relationship.

Note that this curve visually demonstrates Relative
Biologic Effectiveness (RBE) – a comparator of different
particles or photons – which is the ratio of doses giving
identical biologic effect.

For low
doses, the survival curve of x-rays has a
shoulder: reflecting an ability of cells to repair
damage at low dose x-ray exposures (i.e., sub-lethal damage).

By
contrast, the neutron curve does not demonstrate such a shoulder
– thus, RBE is greatest at these lower doses and becomes
constant at higher doses where both dose-response curves are log-linear.